1. Field of the Invention
This invention relates in general to mass storage systems, and more particularly to a method and apparatus for migrating data having a format of a first type to a format of a second type.
2. Description of Related Art
Direct access storage devices (DASDs) are used to store data for host data processing systems. One often used type of DASD is a magnetic disk unit including a number of disks or platters having surfaces with magnetic active material onto which data is written and from which data is read by magnetic read/write heads. In other types of DASDs, optical or other data storage media may be employed.
In a magnetic disk unit, the disks are formatted to define sectors and tracks upon the disk surfaces. Tracks are usually circular regions coaxial with the disk axis where data may be written, and sectors are parts of the tracks capable of storing a predetermined quantity of data written to the disk. Axially aligned tracks on the disks of a DASD are referred to as cylinders. The sectors of a DASD where blocks of data are stored have unique physical data block addresses (DBA), which may be expressed in cylinder, head, record (CHR) notation, relative block address (RBA) notation or in some other way. The disks of the DASD spin in unison around a common axis, and the read/write heads, usually one for each surface, are moved radially in unison across the disk surfaces. When data is read from or written to a physical DBA, the heads are moved into alignment with the cylinder containing the track in which the DBA is found, and the data transfer takes place as the sector or sectors of the DBA spin under the head.
Important considerations in the design and use of DASDs are capacity, speed of data transfer and reliability. For reasons including those discussed in Patterson et al., “A Case for Redundant Arrays of Inexpensive Disks (RAID)”, ACM SIGMOD Conference, Chicago, Ill., Jun. 1-3, 1980, increasing performance levels attained by central processing units (CPUs) and system memory in data processing systems result in the need for larger and faster data storage systems. To achieve these goals of capacity and speed, it has been proposed to use arrays containing a number of DASDs rather than a single very large and expensive DASD for data storage.
To meet the growing requirements of today's computer systems, and simultaneously, to achieve flexibility and reliability in the function and configuration of a mass storage system, manufacturers of such systems have needed to make their systems adaptable to differing host standards. However, it would be desirable to manufacture the same mass storage system for use with the differing standards associated with various host computer manufacturers.
One particular area in which the standards have clearly not been consistent is the data block structure with which a host computer system formats its data. Thus, in an IBM AS/400 system, each block has 520 bytes of data. Other system configurations, such as those using the Unix operating system, require 512 bytes of data per block (the defacto standard). In addition, there can be one or more bytes of error correcting code or parity as required by the host system.
In modern high speed, high capacity systems, the host system does not perform the actual storage of the data on or in the mass storage device. This is left to a sophisticated storage controller which receives the blocks of data and formats them for storage in view of the physical characteristics of a disk drive.
As mentioned above, currently external controllers use a 512 byte sector size on the backing storage disks. However, manufacturers are moving to larger sector sizes to accommodate first sector and system drive information and later full LRC information. However, a problem with migrating a system drive which fully utilizes the available space on the drives which it is built with is that there is no room to allow for a larger sector size. To allow the migration to take place a number of new drives must be added to the system drive to accommodate the additional data which will be is stored.
It can be seen then that there is a need for a method and apparatus that moves data stored in a first (e.g., 512) byte sector format to a second (e.g., 52x) byte sector size.
It can also be seen that there is a need for a method and apparatus that performs data migration without interruption of the host's ability to write and read data from the system.